Project Summary Metastasis, the spread of cancer from primary tumor site to distal organs, is the cause of over 80% of deaths from cancer. The brain is one of the common metastasis locations. Brain metastasis, which develops in the late course of illness, has become a significant problem in clinic due to its rising incidence as a consequence of prolonged survival and limited efficacy of existing systemic therapies. Metastasis is a multi-step process that requires the complex interplay between cancer cells and the stromal cells, a process commonly referred to as ?seed and soil hypothesis? coined over a century ago. The ?soil?, the microenvironment, not only decides the outgrowth of metastatic cancer cells, but also contributes to therapy resistance. The ?seed?, the invaded cancer cells, directly modifies the surrounding brain stromal cells. Our long-term goal is to dissect the complex interactions between cancer cells and brain stromal cells during metastasis. We have developed novel in vivo and in vitro models to address the gaps in our understanding of brain microenvironmental control of metastatic outgrowth. Our data implicate that astrocytes, the unique and most abundant brain cells, activate PPAR? signaling in brain metastatic melanoma cells. Through an integrative approach using in vitro co-culture assays and in vivo brain metastatic mouse models, Aim 1 will delineate the role of PPAR? pathway in melanoma cells during brain metastasis. We will track the dynamic changes and effect of PPAR? signaling in melanoma cells throughout the whole brain metastatic process. Moreover, PPAR? antagonist will be used in our pre-clinical mouse models to test its potential to treat melanoma brain metastasis. Aim 2 will address how astrocytes activate PPAR? signaling in the invades melanoma cells. We hypothesize that astrocytes serve as ?donor? of unsaturated fatty acids, natural agonist of PPAR?, to the invaded melanoma cells. The gained insights may enable us to mechanistically deconstruct melanoma brain metastasis and develop new treatment strategies for patients with little clinical recourse.